The use of honeycomb core structures to act as lightweight structural members in fiber reinforced composite articles is well known. These structures comprise a honeycomb core of aluminum, phenolic or Nomex.RTM. or other open cellular material sandwiched between layers of fiber reinforced resin matrix materials. These structures are laid up with the resin matrix in the uncured or B-stage state, which must be heated, typically under vacuum and pressure, to cure the resin and form the finished composite article.
Presently, these composites use honeycomb cores which are about one inch or less in thickness. The pressure and vacuum required in curing such composite articles creates minimum problems with reference to the cores of this size. However, recently it has become desirable to utilize honeycomb cores with thicknesses greater than one inch. Unfortunately, the use of honeycomb cores of this thickness have created problems during the curing cycle when the composite part is placed under the required vacuum and pressure. It has been determined that the pressure, which may be as high as 50 psi or more, applied to these structures causes the sides of the honeycomb core to collapse and in collapsing it becomes dimensionally distorted. This may be seen clearly in FIGS. 1 and 2 wherein FIG. 1 is the prepreg lay-up composite 1 prior to being cured, using unstabilized honeycomb core 10 with graphite fiber layers 20 on the top and bottom of the honeycomb 10. FIG. 1 shows the cured composite showing the distortion resulting from the curing process. This distortion is typically known as "lemon seeding"; wherein the sides 30 of the structure are caused to collapse under the curing conditions.
A number of approaches have been taken to attempt to stabilize the honeycomb and prevent it from collapsing during the curing process. One such approach was to fill the outermost portions of the honeycomb with a rigid adhesive such as Epocast.TM. epoxy adhesive, but this did not work as it also crushed the honeycomb when the lay-up was cured and additionally added weight to the structure. An alternate approach was to apply a layer of several different materials to see which material, if any, would result in a stabilized article. A few of the materials which were tried were metal bond, fiber glass, and Kevlar.RTM. fibers. However, none of these resulted in a satisfactory solution as the honeycomb continued to collapse. Additionally, a number of the materials which were tried, would not have been desirable from the point of view that they were parasitic, and were not necessarily used in the construction of the particular composite article, and added undesirable material cost and weight to the article.
Therefore, what is required in this art is a simple, inexpensive method for stabilizing the thick honeycomb cores during this curing process.